1. Field of the Technology
The present technology relates to a developing device.
2. Description of the Related Art
In an electrophotographic image forming apparatus, for example, a toner contained in a two-component developer (hereinafter simply referred to as a “developer”) is supplied from a developing device to an electrostatic latent image on a surface of a photoreceptor, thereby a toner image is formed, and the toner image is further transferred and fixed on a record medium, thereby an image is formed. A toner supply from the developing device to the electrostatic latent image is carried out via a developer bearing member from a developing tank which is a container for housing the toner therein. The electrophotographic image forming apparatus is in heavy usage for a copy machine, a printer, a facsimile apparatus, or the like, and is widely spread because high quality images can be formed by easy operation, and maintenance management is facilitated. According to this, further improvement of performance of the electrophotographic image forming apparatus is required. Among them, it is especially important to increase the image forming speed and increase a number of images formed in unit time. There are various kinds of problems with respect to the high image forming speed, for example, prolongation of a life of a carrier contained in the developer. The developer contains a toner and a carrier. The carrier has a function to charge the toner by being agitated and rubbed with the toner in the developing device. A high quality image can be formed by appropriately charging the toner. Further, in general, the toner is sequentially replenished in response to a situation of toner consumption on one hand, the carrier is used until the durable life of the carrier is finished on the other hand. A toner consumption amount is increased under a high image forming speed, so that the carrier is always exposed to rubbing contact with the toner, thereby physical and mechanical stress applied to the carrier are extremely large. As a result, deterioration of the carrier is progressed, so that a uniform charge amount cannot be applied to the toner, causing the charging defect of the toner, thereby the image density of the formed image is instable. Also, the durable life of the carrier is finished within a short term, thereby causing the problem that frequency of the maintenance management to exchange the carrier is increased.
In order to solve such problems, for example, a measure may be adopted in which the size of the developing device is enlarged so as to increase the housed amount of the carrier. This is to lower the frequency at which the carrier is exposed to the stress by the agitation when the image is formed at high speed by housing more carrier in the developing device, so that the usable life of the carrier is intended to be prolonged. However, due to a structural limitation of the image forming apparatus, the size of the developing device cannot be enlarged to a degree corresponding to the high image forming speed in many cases. Also, improvement of the developing tank is attempted. In a longitudinal direction of the developing device, it is general to dispose the developing tank and the developer bearing member, which are formed to have almost same longitudinal dimensions, in parallel with each other. In the technical improvement, a constitution is adopted in which one end portion of the developing tank is extended, a part of the developing tank facing the developer bearing member is made to be a developing region, an extended part is made to be an agitating region, and the developing region and the agitating region are separated from each other by a partition wall member. In an inner space of the developing tank, two screw members, which are agitating and conveying members, are provided in parallel with each other in a longitudinal direction of the developing tank with penetrating the partition wall member, and the toner is replenished into the agitating region through a toner replenishing port formed on the developing tank wall of the agitating region. In the developing tank, by rotations of the screw members, after the newly replenished toner is mixed with the developer in the agitating region, the developer is conveyed from the agitating region toward the developing region in the longitudinal direction of the developing tank. Further, after the developer is in contact with the developer bearing member in the developing region, the developer is conveyed from the developing region toward the agitating region in the longitudinal direction of the developing tank. That is, a conveying path is formed, where the developer, in which the toner and the carrier are uniformly mixed with each other, is conveyed back and forth in the longitudinal direction of the developing tank. By conveying the developer in the longitudinal direction of the developing tank, even in a case where the toner consumption amount is large under the high image forming speed, the toner is rubbed with the carrier over a relatively long distance, so that the sufficient charge is applied to the toner. The physical stress, the mechanical stress, or the like, applied to the carrier is also reduced, so that an exchange period of the carrier can be prolonged. However, even in such a constitution of the developing tank, the toner density tends to be non-uniform in the longitudinal direction of the developing tank, so that there is a fear that the image density of the formed image is lowered.
Also, in order to prevent the charging defect of the toner by the deterioration of the carrier, the trickle system developing tank is suggested. In the trickle system, the toner and the carrier are replenished in the developing tank, and the redundant developer is discharged from the developing tank. Thereby, the deteriorated carrier is gradually exchanged with the new carrier, so that a possibility that the charging defect of the toner is caused is low, even when an operation causing a large stress such as the image forming at high speed is carried out. However, under the conventional trickle system, it is difficult to discharge the developer quantitatively and stably outside the developing tank, so that the amount of the developer in the developing tank is varied, thereby there is the fear that the charging defect of the toner, a temporal inadequacy of the toner amount, or the like, is caused.
On the other hand, a developing device in which three screw members are disposed in parallel with each other in the longitudinal direction of the developing tank is suggested (see Japanese Unexamined Patent Publication JP-A 11-133710 (1999), for example). In the developing device according to JP-A 11-133710, two screw members are disposed near a developing roller such that their developer conveying directions are the same as that of each other, further another screw member is disposed below them in a vertical direction. In the developing device, no partition wall member is provided in the developing tank. By the constitution simply disposing the three screw members, in a case where the image forming speed is set at high, it is difficult to uniformly mix the toner supplied to the developing tank and the developer already existing in the developing tank, so that the inadequacy of the toner density, the charging defect of the toner, or the like, tends to be caused.